Botulinum neurotoxins (BoNTs) are produced by Clostridium botulinum and are the most potent toxins known. These toxins are a well-recognized source of food poisoning, often resulting in serious harm or even death of the victims. There are seven structurally related botulinum neurotoxins or serotypes (BoNT/A-G), each of which is composed of a heavy chain (.about.100 KD) and a light chain (.about.50 KD). The heavy chain mediates toxin entry into a target cell through receptor-mediated endocytosis. Once internalized, the light chain is translocated from the endosomal vesicle lumen into the cytosol, and acts as a zinc-dependent protease to cleave proteins that mediate vesicle-target membrane fusion (“substrate proteins”).
These BoNT substrate proteins include plasma membrane protein syntaxin, peripheral membrane protein SNAP-25, and a vesicle membrane protein synaptobrevin (Syb). These proteins are collectively referred to as the SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins. Cleavage of SNARE proteins blocks vesicle fusion with plasma membrane and abolishes neurotransmitter release at neuromuscular junction. Among the SNARE proteins, syntaxin and SNAP-25 usually reside on the target membrane and are thus referred to as t-SNAREs, while synaptobrevin is found exclusively with synaptic vesicles within the synapse and is called v-SNARE. Together, these three proteins form a complex that are thought to be the minimal machinery to mediate the fusion between vesicle membrane and plasma membrane. BoNT/A, E, and C.sup.1 cleave SNAP-25, BoNT/B, D, F, G cleave synaptobrevin (Syb), at single but different sites. BoNT/C also cleaves syntaxin in addition to SNAP-25.
Due to their threat as a source of food poisoning, and as bioterrorism weapons, there is a need to sensitively and speedily detect BoNTs. Currently, the most sensitive method to detect toxins is to perform toxicity assay in mice. This method requires large numbers of mice, is time-consuming and cannot be used to study toxin catalytic kinetics. A number of amplified immunoassay systems based on using antibodies against toxins have also been developed, but most of these systems require complicated and expensive amplification process, and cannot be used to study toxin catalytic activity either. Although HPLC and immunoassay can be used to detect cleaved substrate molecules and measure enzymatic activities of these toxins, these methods are generally time-consuming and complicated, some of them require specialized antibodies, making them inapplicable for large scale screening. Therefore, there is a need for new and improved methods and compositions for detecting BoNTs.
In FRET assays, two fluorigenic amino acid derivatives are used to replace two native amino acids in a very short synthetic peptide (20-35 amino acids) that contain toxin cleavage sites. The fluorescence signal of one amino acid derivative is quenched by another amino acid derivative when they are close to each other in the peptide, this mechanism is called “Förster resonance energy transfer” (FRET). Cleavage of the peptide separates the two amino acid derivatives and a decreases in FRET can be detected.
FRET assays have been successfully used for detecting BoNTs. (See e.g., US Pat. App. No. 2004/0191887 to Chapman, filed Oct. 28, 2003, US Pat. App. No. 2006/0134722 to Chapman, filed Dec. 20, 2004, U.S. Pat. No. 7,208,285 to Steward (April 2007), and U.S. Pat. No. 7,183,066 to Fernandez-Salas (February 2007), each of which is incorporated herein by reference in their entirety. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply).
Although some success has been demonstrated in applying FRET assays to detection of BoNTs, the sensitivity and specificity are not sufficient. Improved apparatus, systems and methods are therefore needed.